A cold-adapted (ca) in influenza B reassortant virus constructed from the ca donor of Maassab was attenuated in the upper and lower respiratory tract of chimpanzees. Each of the isolates from these primates retained the temperature sensitivity phenotype after replication in the chimpanzee. These observations indicated that influenza B ca reassortants were attenuated and stable genetically in chimpanzees and should be studied further in human volunteers. Infection by vaccinia-respiratory syncytial virus (RSV) glycoprotein recombinants induced a high level of resistance to RSV challenge in the upper and lower respiratory tract of owl monkeys. This indicated that intradermal infection by a vaccinia-RSV recombinant can provide a high level of resistance to a virus that replicates on respiratory tract mucosa and offers encouragement for the ultimate success of vaccinia-RSV recombinants as vaccines for humans. A vaccinia TK+ recombinant virus was markedly attenuated in dermal virulence for patas monkeys when the lymphokine IL-2 was co- expressed. A TK- vaccinia recombinant that co-expressed IL-2 was also less virulent than a TK- recombinant that did not co-express IL-2. Significantly this attenuation effected by IL-2 did not reduce immunogenicity appreciably. SIV and STLV could not be detected in suspensions of parainfluenza virus type 3 and various rotaviruses that had a history of passage in monkey kidney tissue culture and are now being considered for use in humans. Absence of contamination of the candidate vaccine viruses are free of contamination by these two horizontally transmitted retroviruses it is reasonable to continue our evaluation of the candidate vaccines in human volunteers.